1. Field of the Invention
The invention relates to a carbon dioxide absorbent and a method of manufacturing the absorbent and relates particularly to a carbon dioxide absorbent with improved porosity and a method of manufacturing the carbon dioxide absorbent with improved porosity.
2. Description of the Related Art
Carbon dioxide gas exhausted out of a combustion apparatus for burning fuel mainly containing hydrocarbons of a power generator has a temperature as high as about 300° C. or higher at a discharge gas releasing part which is at a site suitable for recovery.
On the other hand, as a conventionally well-known method of separating carbon dioxide gas are a method of using cellulose acetate and a chemical absorption method of using an alkanol amine type solvent. However, the above-mentioned separation methods require the temperature of a gas to be introduced to be about 200° C. or lower. Accordingly, it is needed for the discharge gas for reheating at a high temperature such as carbon dioxide gas discharged out of a combustion apparatus to be cooled to about 200° C. or lower by a heat exchanger, a cooling tower, or the like. Therefore, the energy consumption amount for separating carbon dioxide gas is high.
In such a carbon dioxide gas separation method, Jpn. Pat. Appln. KOKAI Publication No. 9-99214 discloses use of a lithium zirconate-containing carbon dioxide gas absorbent and Jpn. Pat. Appln. KOKAI Publication Nos. 2000-262890 and 2001-170480 disclose use of lithium silicate-containing carbon dioxide gas absorbents. These lithium zirconate- or lithium silicate-containing carbon dioxide gas absorbents are capable of absorbing carbon dioxide gas in a temperature range from room temperature to about 500° C. and release carbon dioxide gas when being heated at a temperature of about 600° C. or higher. Further, these carbon dioxide gas absorbents have an advantageous point that they can repeat absorption and desorption. Further, there is described that addition of carbonates of alkali metals selected from lithium, sodium, and potassium to these carbon dioxide gas absorbents accelerates the absorption reaction of carbon dioxide.
However in the case absorption and desorption of carbon dioxide gas by these carbon dioxide gas absorbents are repeated, the absorption amount of carbon dioxide gas is gradually decreased and it consequently becomes difficult to maintain a stable carbon dioxide absorption characteristic for a long duration.